Conventionally, a cleaning member with a skin layer for use in cleaning by chemical mechanical polishing (hereinafter referred to as CMP) process is known. Such a cleaning member is manufactured by molding the member, cutting the surface to be in contact with a substrate, and forming a skin layer on the surface that has been in contact with the inner surface of the mold (i.e., the side surface of the cleaning member). The skin layer has continuous pores having smaller diameters than those in the inner part made of sponge and also has a lower porosity than the inner sponge part. Therefore, using the cleaning member with such a skin layer for cleaning can hold sufficient water inside while preventing the liquid inside from leaking out of the surface, which enables the cleaning member to have desired water absorbing properties and water supplying properties during cleaning process. In addition, this cleaning member advantageously does not produce particles substantially and has a long life in general. For example, Japanese Patent Laid Open No. 2011-56382 discloses a sponge member having a shaft and a skin layer.
If a cleaning member with a skin layer on the side surface is used for cleaning by CMP, the skin layer on the side surface may be situated between the cleaning member and a substrate during cleaning process and cause contact contamination on the substrate (in such case, there might be resulting in cleaning irregularities in the circumferential direction. See FIG. 12).
On the other hand, if a cleaning member without a skin layer on the side surface is used, cleaning liquid collide with the bare side surface and go into the cleaning member. As a result, particles in the cleaning liquid may go into the cleaning member and then go down to a substrate to adhere to it. In addition, the cleaning member without a skin layer may not have strength enough to keep the cleaning properties under specific circumstances. Therefore, to avoid an occurrence of contact contamination using the cleaning member without a skin layer for cleaning a substrate seems to be preferable, but from other point of view, the cleaning member without a skin layer seems to be not preferable.